The transversus abdominis muscle (Figs. 72-2 and 72-3C) arises from the fascia along the iliac crest and

inguinal ligament and from the lower six costal cartilages and ribs, where it interdigitates with the

lateral diaphragmatic fibers. The muscle bundles of the transversus abdominis for the most part run

horizontally. The lower medial fibers, however, may continue in a more inferomedial course toward the

site of insertion on the crest and pecten of the pubis.

The aponeurosis of the transversus abdominis joins the posterior lamina of the internal abdominal

oblique, forming above the umbilicus a portion of the posterior rectus sheath. Below the umbilicus, the

transversus abdominis aponeurosis is a component of the anterior rectus sheath. The gradual

termination of aponeurotic tissue on the posterior aspect of the rectus abdominis forms the arcuate line

(of Douglas) (Fig. 72-6). The medial aponeurotic fibers of the transversus abdominis insert on the

pecten pubis and the crest of the pubis to form the falx inguinalis. These fibers infrequently are joined

by a portion of the internal oblique aponeurosis; only then is a true conjoined tendon formed.4

The arch formed by the termination of the aponeurotic fibers of the transversus abdominis is called

the aponeurotic arch (Fig. 72-6). The area beneath the arch varies. A high arch may be a predisposing

factor in direct inguinal hernia. Contraction of the transversus abdominis causes the arch to move down

toward the inguinal ligament in a kind of shutter mechanism, which reinforces the weakest area of the

groin when intra-abdominal pressure is raised.

1915

Figure 72-3. A: External oblique muscle and aponeurosis. B: Internal oblique muscle and aponeurosis. C: Transversus abdominis

muscle and aponeurosis. D: Lower rectus abdominis and pyramidalis muscle. The linea alba is formed by the intermeshed fibers of

the aponeuroses of the lateral muscle layers; it is tensed by the pyramidalis, which inserts into it.

Rectus Abdominis

The rectus abdominis (Figs. 72-3D and 72-5) forms the central and anchoring muscle mass of the

anterior abdomen. The rectus muscle arises from the fifth to the seventh costal cartilages and inserts on

the pubic symphysis and pubic crest. Each rectus muscle is segmented by tendinous intersections at the

levels of the xiphoid process and the umbilicus and at a point midway between these two. The principal

blood supply reaches the muscle from the superior and inferior epigastric arteries (Fig. 72-5), which

anastomose just superior to the umbilicus. Other vessels are anterior branches of the intercostal arteries;

these reach the muscle by entering the lateral aspect of the rectus sheath. The innervation of the muscle

is from the 7th to the 12th intercostal nerves, which laterally pierce the aponeurotic sheath of the

muscle. The lateral edge of the muscle is demarcated by a slight depression in the aponeurotic fibers

coursing toward the muscle; this depression is the semilunar line.

Figure 72-4. Ligamentous structures of the inguinal region. The iliopubic tract is not seen in this view because it is obscured by the

inguinal ligament. The lacunar ligament is the expanded medial end of the inguinal ligament; on the pecten pubis, it blends with

the inguinal (Cooper) ligament.

The small pyramidalis muscle (Fig. 72-3D) accompanies the rectus abdominis at its origin in a

minority of people. The pyramidalis arises from the pubic symphysis. It lies within the rectus sheath and

tapers to attach to the linea alba, the conjunction of the two rectus sheaths and the major site of

insertion of three aponeuroses from all three lateral muscle layers.

Rectus Sheath

Although the components of the rectus sheath individually have been discussed in relation to the three

lateral abdominal muscles, it should also be considered as a distinct entity. Three features of the rectus

muscle and its sheath can be observed even topographically in well-muscled or very thin subjects:

1. The semilunar line is a slight depression in the aponeurotic fibers corresponding to the lateral edge of

the rectus muscle. It marks the site of initial lateral insertion of the aponeurotic tendons of the lateral

abdominal muscles.

2. The tendinous inscriptions divide each muscle into three parts.5 These are the basis of the expression

“six pack,” popularized by bodybuilders.

3. The linea alba is the midline confluence of the aponeuroses of the rectus muscles and also the internal

and external oblique muscles.

1916

Figure 72-5. A: Immediately superior to the umbilicus, the rectus sheath consists of anterior and posterior components. The

anterior sheath is composed of the aponeuroses of the external and internal abdominal oblique muscles, and the posterior sheath

consists of the posterior aponeurotic lamina of the internal oblique and the aponeurosis of the transversus abdominis muscle. B:

The rectus sheath inferior to the arcuate line (of Douglas) consists of an anterior portion made up of fibers from all aponeurotic

layers; the posterior portion at this point comprises only transversalis fascia covered internally by peritoneum.

The composition of the rectus sheath varies depending on the level sampled. The anterior sheath

superior to the umbilicus is composed of the aponeurosis of the external abdominal oblique and the

anterior lamina of the internal abdominal oblique. The transversalis aponeurosis does not participate in

the formation of the anterior sheath at this level. At a variable level inferior to the umbilicus, the

anterior sheath is a composite of all the aponeurotic layers.

The posterior sheath of the rectus muscle superior to the umbilicus is a lamination of the posterior

lamina of the aponeurosis of the internal abdominal oblique and the transversus abdominis aponeurosis.

The external abdominal oblique does not participate in the formation of the posterior portion of the

rectus sheath. At a highly variable site inferior to the umbilicus, all the aponeurotic tendons pass

anteriorly to form the anterior rectus sheath. The fibers of the posterior sheath are seen to attenuate

gradually. The aponeurotic fibers do not end abruptly at the arcuate line. This transfer of connective

tissue away from the posterior rectus sheath causes the arcuate line (of Douglas) to form on the

posterior surface of the muscle (Fig. 72-6). The tissue covering the deep surface of the rectus muscle

inferior to the arcuate line is primarily the transversalis fascia.

Some have questioned this traditional scheme of rectus sheath composition, contending that each of

the aponeurotic layers superior to the umbilicus is actually bifid, with both contributing to the anterior

and posterior sheaths.6 The fibers of the posterior sheath are seen to attenuate gradually. The concept of

rectus sheath composition favored by most is shown in Figure 72-7.7

Innervation and Blood Supply of the Anterior Abdominal Wall

The innervation of the anterior wall muscles is multiple. The lower intercostal and upper lumbar nerves

(T7 to T12, L1, L2) contribute most of the innervation to the lateral muscles, the rectus abdominis, and

the overlying skin. The nerves pass anteriorly in a plane between the internal abdominal oblique and

the transversus abdominis, eventually piercing the lateral aspect of the rectus sheath to innervate the

muscle therein. The external oblique muscle receives branches of the intercostal nerves, which penetrate

the internal oblique. The anterior ends of the nerves form part of the cutaneous innervation of the

abdominal wall. The first lumbar nerve divides into the ilioinguinal and iliohypogastric nerves. These

may divide within the psoas major muscle or between the internal oblique and transversus abdominis

muscles. The ilioinguinal nerve may communicate with the iliohypogastric nerve before innervating the

internal oblique. The ilioinguinal nerve then passes through the external inguinal ring to run with the

spermatic cord, whereas the iliohypogastric nerve pierces the external oblique to innervate the skin

above the pubis. The cremaster muscle fibers, which are derived from the internal oblique muscle, are

innervated by the genitofemoral nerve (L1, L2).

1917

Figure 72-6. The deep inguinal region, pelvis, and anterior abdominal wall from the viewpoint of a surgeon using a laparoscopic

technique. The anterior wall folds upward approximately at the iliopubic tract in this illustration.

Figure 72-7. Patterns of midline decussation of the aponeuroses. A: Single anterior and posterior lines of decussation. B: Single

anterior and triple posterior lines of decussation. C: Triple anterior and posterior lines of decussation.

The blood supply of the lateral muscles of the anterior wall is primarily from the lower three or four

intercostal arteries, the deep circumflex iliac artery, and the lumbar arteries. The rectus abdominis has a

complicated blood supply derived from the superior epigastric artery (a terminal branch of the internal

thoracic, or internal mammary, artery), the inferior epigastric artery (a branch of the external iliac

artery), and the lower intercostal arteries. The latter arteries enter the sides of the muscle after

traveling between the oblique muscles. The superior and inferior epigastric arteries enter the rectus

sheath and anastomose near the umbilicus.

1918